1. Field of the Invention
The present invention relates to coal desulfurization and, more particularly, to an improved coal pretreatment process for separating organic and inorganic sulfur components of coal.
2. Description of the Prior Art
The U.S. reserve of coal is about 3 trillion tons. Although the most abundant (80%) fossil fuel in America is coal, the U.S. consumption pattern is quite a reversal of form in terms of utilization, with coal representing only 17%, oil and gas about 78%.
The demand for all the fossil fuels combined is expected to double by the year 2000, even with the increasing use of nuclear power. While the domestic supply of crude oil and natural gas is not likely to keep apace with the energy demand, coal can play an important role in filling such a gap and thus reduce the requirements for imported supplies of oil and gas.
Coal, the fossilized plant life of prehistoric times, contains various amounts of sulfur due to the nature of its origin. Under most existing commercial technology, the generation of electricity from coal poses environmental problems because of sulfur oxides and particulate emissions. Since most of the coals in this country, particularly the Eastern and Midwestern coals, have high sulfur content (&gt;2%) there is a need for an economical process of converting high sulfur coals to clean fuel (&lt;1.2 lbs. of SO.sub.2 emission per million BTU thermal output by EPA standard) to utilize coal as a source of energy without causing serious air pollution. So the need for converting massive coal reserves to clean-burning solid fuel, liquid fuel and pipeline quality gas is self evident. If the vast coal reserve is converted to clean fuel, it can supply most of the energy needs of the United States for the next three centuries.
At the present time, about one-half of the electric power in the United States is generated from natural gas and petroleum; most of the other half is from coal. If the coal is converted to clean fuel for electric utilities, petroleum and natural gas would be released for other essential uses, especially as a starting material for the synthetic rubber and plastics industry.
Sulfur in coal occurs in two types, generally in approximately equal amounts (50%): inorganic sulfur primarily as pyrites with minor amounts of sulfates and organic sulfur in the forms of thiophene, sulfide, disulfide and mercaptan chemically bound in the organic structure of coal.
Presently, sulfur is removed in post-combustion processes by stack gas scrubbing to remove sulfur oxides generated during combustion. However, the existing flue-gas desulfurization processes are expensive processes and produce large amounts of sludge. Physical separation methods only remove the inorganic sulfur components. Hydrodesulfurization processes which remove sulfur from the fuel before combustion are used extensively in petroleum desulfurization and are being considered in many coal conversion processes under development.
Hydrodesulfurization of coal requires large amounts of hydrogen and/or other raw material directly derived from petroleum, and requires large capital costs for equipment. Typically reactor temperatures are from 350 to 450.degree. C., and pressures range from 1000 to 4000 PSIA when catalysts are employed, or even more drastic process conditions for non-catalytic systems. Even when reliable systems are achieved, the high temperatures required serve to maximize corrosion and as a result, process reactors require major overhaul every several months or at least once a year, depending on the severity of treating. A low temperature desulfurization process with minimum capital investment costs thus has long been desired.